The present invention relates to copper-base alloys for use mainly in leadframes of semiconductor devices.
Semiconductor chips having integrated circuits are usually connected to leadframes and packaged in molded resins with only leadframe terminals exposed, whereby IC devices are provided. Leadframe materials which may be used for such integrated circuit devices are, in general, required to have the following properties:
(1) Good electric and thermal conductivities
Since leadframes serve as electric conductors for supplying electric signals to the circuits, they should have good electric conductivity. Also, since heat generated in the circuits should be dissipated through the leadframes, they should have good thermal conductivity, too.
(2) High mechanical strength
When the leadframe terminals of semiconductor devices are inserted into sockets of circuit boards, misalignments of the terminals and the sockets may occur. Accordingly, the leadframe terminals should be tough enough not to be bent even when pressed under misaligned conditions. Further, they should have high resistance to fatigue by repeated bending.
(3) Good heat resistance (high softening temperature)
During the production of semiconductor devices, die bonding, wire bonding and resin molding are performed, exposing leadframes to high temperatures of 300.degree.-450.degree. C. Accordingly, leadframes must have good heat resistance, namely high softening temperatures so that they can maintain high mechanical strength even at such high temperatures.
(4) Coefficient of thermal expansion close to those of semiconductor chips and molded resin packages
If there is large difference in coefficient of thermal expansion between leadframes and semiconductor chips and molded resin packages, distortions would occur during the assembling steps involving heating. Such distortions might cause the variations of semiconductor chips' characteristics and the deteriorations of the adhesion of the leadframes to resin packages. In order to prevent these problems, the leadframes should have thermal expansion coefficients as close as possible to those of the semiconductor chips and the molded resin packages.
(5) Good platability
Leadframes are plated with gold or silver on their surfaces which are subjected to die bonding with the semiconductor chips. Therefore, leadframes should have good affinity to platings. That is, platings should be strongly adhered to the leadframe surfaces and should have as small defects as possible.
(6) Good solderability
Leadframe terminals are coated with tin or soldered in advance to facilitate soldering thereof on the circuit boards. Accordingly, leadframes should have good solderability, namely they should be highly wettable with solders.
(7) Good solder durability
Semiconductor devices soldered on circuit boards should not deteriorate their characteristics during their entire life. In general, soldered portions are one of those vulnerable to deterioration. Therefore, the soldered portions of the leadframes should be able to withstand any possible environment in which semiconductor devices are used, without deteriorating the adhesion thereof to solder layers. Such property is called herein "solder durability."
(8) Good adhesion to molded resin packages
In general, most semiconductor integrated circuit devices are packaged with molded resins. Accordingly, leadframes are required to have good adhesion to molded resins.
Typical alloys conventionally used for leadframes are iron-nickel alloys such as Fe-42% Ni alloys and Fe-29% Ni-17% Co alloys, and copper base alloys such as Ni-containing copper alloys and phosphor bronze. These Fe-base alloys have excellent mechanical strength, but their electric and thermal conductivity is not always satisfactory. On the other hand, the copper-base alloys have good electric and thermal conductivity, and are much less expensive than the iron-nickel counterparts. However, they lack mechanical strength.
Nevertheless, because of their advantages of electric, thermal conductivity and cost, the copper-base alloys have recently been finding increasing wider use in leadframes. Thus, various attempts have been made to improve their mechanical strength, heat resistance and other properties.
U.S. Pat. No. 4,249,941 to R. Futatsuka, et al. discloses copper-base alloys for leadframes of integrated circuit devices consisting essentially of 0.5-1.5 weight % Fe, 0.5-1.5 weight % Sn, 0.01-0.35 weight % Sn, 0.01-0.35 weight % P and balance Cu and inevitable impurities. They are, however, not necessarily satisfactory in terms of mechanical strength and solder durability.
U.S. Pat. No. 4,337,089 to K. Arita, et al. discloses copper-nickel-tin alloys for leadframes containing 0.5-3.0 weight % Ni, 0.3-0.9 weight % Sn, 0.01-0.2 weight % P, 0-0.35 weight % Mn and/or Si, and balance Cu. These alloys have good tensile strength but relatively poor electric conductivity. In addition, they do not have sufficient solder durability.
There are some other copper-base alloys: Copper Alloy C 194 (2.35% Fe-0.03% P-0.12% Zn), Copper Alloy C 195 (1.5% Fe-0.1% P-0.8% Co-0.6% Sn), and Copper Alloy C 155 (0.06% P-0.07% Ag-0.11% Mg).
These copper-base alloys, however, do not necessarily have a preferable combination of good electric conductivity and high mechanical strength. That is, those having good mechanical strength have relatively poor electric conductivity and vice versa. And full attention has not been paid to solderability. For instance, some of alloy elements added to improve the alloy's mechanical strength largely tend to deteriorate its solder wettability and solder durability in a way or another. This extremely lowers reliability of semiconductor devices, and it may be detrimental particularly for applications in which the leadframes are somewhat exposed to high temperatures.